The present invention relates generally to control of a power transmission in a work vehicle such as an agricultural tractor. More particularly, the invention relates to an improved flow restrictor assembly for reducing clutch fill rates for clutches incorporated in such transmissions.
In the field of transmission systems for work vehicles, such as agricultural tractors, a number of transmission configurations and control schemes have been proposed and are presently in use. Such transmissions typically include a collection of intermeshing gears either fixed to transmission shafts or rotating freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between an engine output shaft and a transmission output shaft to transmit engine torque at a desired speed to driven wheels of the vehicle. Control systems for commanding engagement of the clutches typically include electronic circuitry that responds to operator controls, such as an upshift/downshift lever, a forward/reverse lever and the like in the vehicle cab. The control system sends electric signals to hydraulic valves that channel pressurized fluid to the clutches. The control systems thus cause the clutches to engage and disengage in predetermined combinations to accelerate, decelerate and drive the vehicle as desired by the operator. Transmissions and control systems of this type are described in U.S. Pat. No. 4,425,620, issued on Jan. 10, 1984 and assigned to Steiger Tractor, Inc. at issuance, and U.S. Pat. No. 4,967,385, issued on Oct. 30, 1990 and U.S. Pat. No. 5,902,344, issued on May 11, 1999, both assigned to Case Corporation at issuance.
Direct shifting between gears is often provided for in transmissions such as those described above. This process, called "power shifting" involves disengaging a first set of one or more clutches (the "off-going clutches") while substantially simultaneously engaging a second set of one or more clutches (the "on-coming clutches"). This is especially valuable when a work vehicle is in forward motion, attached to an implement. Both first and second sets of clutches cannot be engaged at the same time (called "lockup") with the engine rotating the transmission input shaft without causing serious wear and possible damage. On the other hand, if the off-going clutches are disengaged long before the on-coming clutches are engaged, the tractor will be disengaged entirely for a period of time, and will decelerate. This phenomenon is called a "torque-hole" in industry parlance. The optimum situation is to begin engaging the oncoming clutches while the off-going clutches are being disengaged. Thus, as torque applied by the off-going clutches decreases, torque applied by the on-coming clutches increases. This overlap of torques eliminates the torque hole and provides a smoother transition between gears.
It is difficult to properly overlap the torques due to inherent and changeable time delays during clutch engagement and disengagement. Hydraulic clutches must be filled with hydraulic fluid for an initial period ("fill time") before they begin engaging and transmitting torque. Furthermore, clutch valves have a built-in lag from the time they are energized to the time they open and permit fluid to flow. The results is an inherent delay between the time the on-coming clutch valves are signaled to operate, and the time torque begins to be transmitted by the on-coming clutch. One solution to this problem is to monitor the pressure in each clutch as the clutch fills in order to determine the point of incipient engagement, which appears as a sudden pressure spike in the clutch. Measuring the pressure in each clutch requires a pressure transducer for each clutch, however. This adds to the complexity of the system and the likelihood of component failure.
There is a need, therefore, for an improved apparatus for reducing clutch fill times in a controlled manner, especially for clutches in power shift transmissions. In particular, there is a need for an apparatus adapted to limit the rate of flow of hydraulic fluid to clutches to smooth the engagement and disengagement of the clutches. More particularly, there is a need for an apparatus that can modulate clutch fill times while a vehicle is being operated.